Hi Juliette,

Thank you for reaching out to the UVM Master Gardener Program with your question. The reason lettuce bolts is that if growing conditions aren’t perfect, the plant can quickly go to seed in order to produce the next generation of lettuce. “Bolting in plants occurs when an internal signal turns on the production of flowers and fruits. In lettuce and spinach the process of flowering also triggers the production of chemicals that produce the bitter taste. It was long thought that these plants bolted in the heat of summer. That is, they were triggered to flower by high temperatures. It has been shown experimentally that lettuce and spinach actually take their cues for flowering from the length of the hours of darkness at night. They are triggered to flower when the length of the night gets short. Since temperatures also go up as the days get longer most gardeners tend to blame the bolting on increasing temperatures.

Lettuce and spinach can be kept from bolting by completely excluding light each night for a long enough period to prevent the release of the flowering triggering substances. That is difficult to do in the home garden. Shade cloth that reduces the quantity of light during the day does not increase the critical length of night, and will have little effect on bolting. (Dave Orbin, Penn State Master Gardener, Luzerne County)“

Most articles recommend that you plant lettuce early in the spring or towards the fall in order to take advantage of when lettuce grows best. 12 hours of direct sun in the heat of the summer can stress the lettuce and make it tend to bolt. As the summer heat develops and the days get longer there are strategies that people use to keep a continuous supply of lettuce. One is to plant lettuce varieties that are slow to bolt every 2 weeks throughout the growing season. Then as the season goes on, it is suggested to plant them in an area that is shaded for part of the day. Lettuce likes a temperature of about 60 to less than 80 degrees.

https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/426/426-408/SPES-253.pdf

Another thing to keep in mind is that you should make the growing conditions favorable for the lettuce to thrive. This article describes how lettuce likes slightly acidic soil that has been amended with organic matter. Note under ‘Seed Facts and Planting’, there are some tips for providing shade in the heat of the summer. They recommend shade cloth rather than a basic row cover - even old sheets can help. https://piedmontmastergardeners.org/article/summer-lettuce-to-grow-or-not-to-grow/

This last article discusses how 30% black shade cloth can keep the temperature low enough to grow lettuce that takes longer to bolt - it doesn’t mention 50% black shade cloth. https://sites.udel.edu/weeklycropupdate/?p=19864 The good thing about a shade cloth is that it can be put on or taken off if the conditions warrant it. I wish you luck with your lettuce and I hope that you find a solution that works for you. Sincerely,